Three-phase synchronous motors without starting cage are by their very principle tied to the frequency of the supplying three-phase system. Such motors cannot therefore be started, i.e. run up to speed, directly from the system. Rather, a run-up enabling device must be provided between the three-phase system and the synchronous machine. For this purpose a frequency converter is normally used. The frequency converter consists of a rectifier, a DC link (capacitor) and an inverter. It is used to produce three-phase current with adjustable frequency.